The Mobile Station International ISDN number (MSISDN) is the standard telephone number that is used to identify a subscriber to a telecommunications network. The MSISDN therefore provides means for uniquely identifying a mobile user of a mobile telecommunications network. The MSISDN is considered by mobile phone operators to be sensitive information. This is because if third parties are able to access the MSISDNs registered with a network, there is a risk that the third party will commence activities such as nuisance calling or sending spam messages to subscribers. Therefore mobile phone operators are reluctant to reveal the MSISDNs of their subscribers to third parties.
The third generation partnership project (3GPP) has recently defined a new concept known as IMS (IP-based Multimedia Subsystem). The IMS is a set of core network servers sitting behind the GGSN (which will be briefly described below) in the packet switched domain. These servers are introduced in order to process signalling between end users. The aim of IMS is to allow bodies such as mobile telephone network operators to provide services to their users as efficiently and effectively as possible. For example, the IMS architecture is likely to support the following communication types, voice, video, instant messaging, “presence” (a user's availability for contact), location-based services, email and the web. Further communication types are likely to be added in the future.
The diverse collection of communications devices requires efficient session management due to the number of different applications and services that will be developed to support these communication types. The 3GPP have chosen Session Initiation Protocol (SIP) for managing these sessions.
The SIP protocol is a session-based protocol designed to establish IP based communication sessions between two or more end points or users. Once a SIP session has been established, communication between these end points or users can be carried out using a variety of different protocols (for example those designed for streaming audio and video). These protocols are defined in the SIP session initiation messages.
With IMS, users are no longer restricted to a separate voice call or data session. Sessions can be established between mobile devices that allow a variety of communication types to be used and media to be exchanged. The sessions are dynamic in nature in that they can be adapted to meet the needs of the end users. For example, two users might start a session with an exchange of instant messages and then decide that they wish to change to a voice call, possibly with video. This is all possible within the IMS framework. If a user wishes to send a file to another user and the users already have a session established between each other (for example, a voice session) the session can be redefined to allow a data file exchange to take place. This session redefinition is transparent to the end user.
For some of the services supported by IMS, in particular instant messaging supported by a third party service provider, it has hitherto been necessary for mobile operators to exchange information concerning the identity of their subscribers, e.g. the MSISDN, with the third party service provider. This is because in such implementations of instant messaging, each user is identified by a respective username that is allocated by the third party service provider. However, because such user names are typically not widely known, the service provider also needs to know each user's MSISDN, as this is much more widely known. This allows users to participate in instant messaging when they only know the MSISDN of their contact (and not the username). The usernames are stored in a look-up table against the MSISDN and possibly other contact details (e.g. email address) on a server which is administered by the third party service provider. However, as explained previously, network operators consider disclosing such sensitive information as the MSISDN to be undesirable. Accordingly, it would be desirable to solve this problem and general problems related to making data available.